Process for producing a cable with a recyclable coating

ABSTRACT

A process for producing a cable, such as, for medium-tension or high-tension power transmission or distribution, having at least one coating layer made of a thermoplastic polymer material. The process includes at least the following steps: (a) extruding a thermoplastic material of at least one thermoplastic polymer and at least one dielectric liquid; (b) passing the thermoplastic material through at least one static mixer; and (c) depositing and shaping the thermoplastic material around a conductor belonging to the cable so as to obtain a layer of thermoplastic coating on the conductor.

[0001] The present invention relates to a process for producing a cable,in particular for medium-tension or high-tension power transmission ordistribution.

[0002] More particularly, the present invention relates to a process forproducing a cable, preferably for medium-tension or high-tension powertransmission or distribution, which has at least one coating layer madeof a thermoplastic polymer material.

[0003] The need for products that are highly environmentally friendly,obtained from materials that do not damage the environment, eitherduring production or during use, and that are readily recyclable at theend of their life, is particularly felt even in the sector of powercables, telecommunications cables, data transmission cables and/orcombined power/telecommunications cables.

[0004] However, the use of environmentally friendly materials isdecidedly conditioned by the need to limit the costs and yet maintainperformances, under the most common working conditions, that areequivalent to or even better than those of conventional materials.

[0005] In the sector of medium-tension or high-tension powertransmission cables, the different coatings, which surround theconductor commonly consist of a crosslinked polymer material based onpolyolefins, in particular crosslinked polyethylene (XLPE) orethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) elastomericcopolymers, which are also crosslinked. The crosslinking, carried outafter the step of extruding the polymer material around the conductor,gives said material satisfactory mechanical performances, even under hotconditions in continuous use and under conditions of current overload.The polymer material is usually crosslinked via a radical route, byadding organic peroxides, or via a silane route, by means of introducingonto the polyolefin chains hydrolysable silane groups which, in thepresence of water and of a suitable condensation catalyst give rise tothe crosslinking.

[0006] However, it is well known that crosslinked materials are notrecyclable, as a result of which both the process wastes and the cablecoating material at the end of its life can be disposed of only byincineration.

[0007] Electrical cables are also known which have an insulationconsisting of a multilayer winding of a paper or paper/polypropylenelaminate impregnated with very large amounts of a dielectric liquid(commonly known as bulk-impregnated cables or oil-filled cables). Bycompletely filling the spaces present inside the multilayer winding, thedielectric liquid prevents the occurrence of partial discharges and thusperforation of the electrical insulation. Dielectric liquids that arecommonly used include products such as, for example: mineral oils,polybutenes, alkylbenzenes and the like (see for example patents U.S.Pat. No. 4,543,207, U.S. Pat. No. 4,621,302, EP-A-0 987 718 and WO98/3213).

[0008] However, it is known that bulk-impregnated cables have manydrawbacks compared with cables with extruded insulation, as a result ofwhich their use is currently restricted to specific fields ofapplication, in particular to the production of high-tension andultra-high-tension direct-current transmission lines, both forterrestrial installations and, above all, for underwater installations.Specifically, the production of bulk-impregnated cables is very complexand expensive, not only because of the high cost of the laminates, butalso because of the difficulties inherent to the winding steps of thelaminate and its subsequent impregnation with the dielectric liquid. Inparticular, the dielectric liquids used are required to have a lowviscosity under cold conditions so as to allow high-speed and uniformimpregnation, and at the same time they are required to have littletendency towards migration during the laying and functioning of thecable, so as to avoid losses of said liquid from the cable ends or dueto breakage. In addition, bulk-impregnated cables are not recyclable andtheir use is limited to working temperatures lower than 90° C.

[0009] In the context of power cables with a non-crosslinked extrudedcoating, the use of thermoplastic materials of different kinds has beenproposed in the prior art.

[0010] For example, patent application WO 96/23311 discloses alow-voltage, high-current cable in which the insulating coating, theinner sheath and the outer sheath are made of the same non-crosslinkedpolymer material, coloured black by addition of carbon black. The use ofthe same material in the different layers does not require theabovementioned components to be separated in a recycling process. For amaximum working temperature of 90° C., the possibility of usingheterogeneous thermoplastic elastomers consisting of a polypropylenematrix in which is dispersed an elastomeric phase consisting of EPR orEPDM copolymers is indicated.

[0011] Patent application EP-0 893 801, in the name of the Applicant,discloses a cable comprising a conductor and one or more coating layers,in which at least one of said coating layers comprises, asnon-crosslinked base polymer material, a blend comprising: (a) acrystalline propylene homopolymer or copolymer; and (b) an elastomericethylene copolymer with at least one α-olefin containing from 3 to 12carbon atoms, and optionally with a diene; said copolymer (b) beingcharacterized by a 200% tension set value (measured at 20° C. for 1minute according to ASTM standard D 412) of less than 30%.

[0012] Patent application EP-A-0 893 802, in the name of the Applicant,discloses a cable comprising a conductor and one or more coating layers,in which at least one of said coating layers comprises, asnon-crosslinked base polymer material, a blend comprising: (a) acrystalline propylene homopolymer or copolymer; and (b) a copolymer ofethylene with at least one α-olefin containing from 4 to 12 carbonatoms, and optionally with a diene; said copolymer (b) beingcharacterized by a density of between 0.90 and 0.86 g/cm³, and by acomposition distribution index, defined as the percentage by weight ofthe copolymer molecules having an α-olefin content which is not morethan 50% of the total average molar content of α-olefin, of greater than45%.

[0013] Patent application WO 00/41187, in the name of the Applicant,discloses a cable comprising a conductor and at least one coating layerbased on a non-crosslinked polymer material comprising a heterogeneouscopolymer having an elastomeric phase based on ethylene copolymerizedwith an α-olefin and a thermoplastic phase based on polypropylene. Theelastomeric phase in said heterogeneous copolymer represents at least45% by weight relative to the total weight of the heterogeneouscopolymer, and the heterogeneous copolymer is substantially free ofcrystallinity derived from polyethylene blocks. The elastomeric phasepreferably consists of an elastomeric copolymer of ethylene andpropylene which comprises from 15% to 50% by weight of ethylene and from50% to 85% by weight of propylene, relative to the weight of theelastomeric phase.

[0014] Patent application WO 99/13477 discloses an insulating materialconsisting of a thermoplastic polymer forming a continuous phase whichincorporates a liquid or readily fusible dielectric, which forms amobile interpenetrating phase in the solid polymer structure. The weightratio between the thermoplastic polymer and the dielectric is between95:5 and 25:75. The insulating material can be produced by hot-blendingthe two components in a batchwise or continuous manner (for example bymeans of an extruder). The resulting blend is then granulated and usedas insulating material for the production of a high-tension electricalcable by means of extrusion around a conductor. The material can be usedeither in thermoplastic form or in crosslinked form. Among thethermoplastic polymers the following are indicated: polyolefins,polyacetates, cellulose polymers, polyesters, polyketones,polyacrylates, polyamides and polyamines. It is particularly suggestedto use polymers of low crystallinity. The dielectric is preferably asynthetic or mineral oil, of low or high viscosity, in particular apolyisobutene oil, naphthenic oil, polyaromatic oil, α-olefinic oil orsilicone oil.

[0015] The Applicant believes, however, that the technical problem ofobtaining an electrical cable with a coating consisting of athermoplastic polymer material which has mechanical and electricalproperties that are comparable with those of cables with an insulatingcoating made of crosslinked material, remains to be solved. Inparticular, the Applicant set itself the problem of producing a cablewith a non-crosslinked insulating coating which has good flexibility andhigh mechanical strength under both hot and cold conditions, and at thesame time high dielectric rigidity, without using potentially pollutantproducts during the life cycle of the cable, that is to say from itsproduction to its disposal.

[0016] In view of the abovementioned problem, the Applicant has foundthat although the addition of dielectric liquids to thermoplasticpolymer materials significantly increases the electrical properties ofthe material (in particular the dielectric rigidity), it presents manyproblems from the point of view of industrial implementation.

[0017] Specifically, the Applicant believes that the dielectric liquidadded to the thermoplastic polymer material needs to maintain theproperties of the material (thermomechanical properties, ease ofhandling) without giving rise to phenomena of exudation of saiddielectric liquid. In addition, the dielectric liquid should distributeitself uniformly within the material, so as to ensure high electricalperformances throughout its thickness. For example, when the coatingmade of thermoplastic polymer material is the insulating coating, it isimportant that the dielectric liquid should distribute uniformly itselfthroughout the coating thickness and should be present, in particular,in the interface zones between the inner and outer semiconductive layersusually present in a medium-tension and/or high-tension electrical powertransmission and/or distribution cable. In this way, the resulting cablecan ensure substantially constant performances over time, and thus ahigh level of reliability, even at elevated working temperatures (of atleast 80° C., preferably of at least 90° C.).

[0018] In particular, the Applicant has found that the action of mixingthe dielectric liquid into the thermoplastic material, which can takeplace during an extrusion process, does not make it possible to obtain acoating with a substantially homogeneous distribution of the dielectricliquid throughout its thickness. Specifically, the dielectric liquidtends to become concentrated in the inner zones of said coating, to thedetriment of the outermost zones, which are actually the zones that aremost sensitive to partial discharges and, thus, to perforation.

[0019] The Applicant has now found that it is possible to produce acable with at least one thermoplastic coating layer which includes adielectric liquid distributed substantially uniformly throughout thethickness of said coating. This is obtained by means of a process whichcomprises extruding a thermoplastic material, comprising a thermoplasticpolymer mixed with a dielectric liquid, and passing the thermoplasticmaterial through a static mixer. Next, the material is deposited arounda conductor by conventional techniques, for example by means of anextrusion head. In this way, a cable is obtained which is suitable inparticular for electrical power transmission and/or distribution, havinga thermoplastic coating of high dielectric rigidity which ensuressubstantially constant performances over time, and thus highreliability.

[0020] In accordance with the present invention, the expression “cableconductor” means a conductive element per se, in elongate form andpreferably made of metal, or a conductive element coated with asemiconductive layer. As stated earlier, the latter solution, whichinvolves the use of a semiconductive layer both inside and outside theinsulating layer, is typically used for electrical cables formedium-tension and/or high-tension power transmission and/ordistribution.

[0021] In a first aspect, the present invention thus relates to aprocess for producing a cable provided with at least one thermoplasticcoating, which comprises:

[0022] extruding a thermoplastic material comprising at least onethermoplastic polymer and at least one dielectric liquid;

[0023] passing said thermoplastic material through at least one staticmixer;

[0024] depositing and shaping said thermoplastic material around aconductor so as to obtain a thermoplastic coating layer on saidconductor.

[0025] According to a first embodiment, said coating layer is a layer ofelectrical insulation.

[0026] According to another embodiment, said coating layer is asemiconductive layer.

[0027] According to a preferred embodiment, the dielectric liquid isadded to the thermoplastic polymer when said polymer is in the moltenstate.

[0028] The extrusion step according to the process of the presentinvention, carried out in an extruder which is known per se, generallycomprises the following sub-steps:

[0029] feeding the thermoplastic polymer into at least one extruder;

[0030] conveying the thermoplastic polymer through said at least oneextruder;

[0031] plasticizing the thermoplastic polymer travelling through said atleast one extruder.

[0032] The addition of the dielectric liquid to the thermoplasticpolymer preferably takes place by injecting the liquid into theextruder, preferably in a zone of the extruder in which the polymer isin molten state, i.e. already plasticized, in particular in a downstreamzone of the extruder. This solution makes it possible to meter out thedielectric liquid accurately and to obtain excellent distribution ofthis liquid in the thermoplastic polymer. At the same time, the additionof the dielectric liquid to the already plasticized polymer ensuresstability to the extrusion process, since the presence of dielectricliquid in the early extrusion steps, when the polymer is not yet molten,can cause irregularities in the movement of the material through theextruder on account of the lubricant action brought about by thisliquid.

[0033] Preferably, the addition of the dielectric liquid to thethermoplastic polymer inside the extruder takes place in at least twoseparate points so as to distribute the dielectric liquid as uniformlyas possible in the thermoplastic polymer.

[0034] According to another embodiment, the dielectric liquid is addedto the thermoplastic polymer when said polymer is in the solid state.

[0035] The addition of the dielectric liquid to the thermoplasticpolymer in the solid state can take place, for example: a) during theabovementioned feeding sub-step; b) before said feeding sub-step, thatis to say before the polymer is fed into the extruder; or c) in a zoneof the extruder in which the thermoplastic polymer is in the solidstate.

[0036] In case b) mentioned above, the addition of the dielectric liquidcan take place, for example, during a prior step of compounding thepolymer in a mixer (of batchwise or continuous type), or by impregnatingthe polymer in the form of granules or powder.

[0037] At the end of the extrusion step and before the step ofdepositing and shaping the coating around the conductor, thethermoplastic material is preferably subjected to a filtration step, soas to remove any impurities, in particular metal particles, which canimpair the electrical properties of this material. The filtration stepcan be carried out between the extrusion step and the step of passingthe thermoplastic material through the static mixer, or can be carriedout between the step of passing the thermoplastic material through thestatic mixer and the step of depositing and shaping the material aroundthe conductor. The filtration step can be carried out by using knowndevices, for example mesh filters or the like.

[0038] The static mixer which can be used in the process according tothe present invention is generally a blending device, which is known perse in the art, containing no moving parts, in which the blending actionis obtained by forcing of the material to be blended past stationaryblending elements. By diverting the direction of the flow orconstraining this flow to pass through preferred channels, said blendingelements carry out numerous subdivisions and recombinations of the flow,thus making it possible to obtain the desired uniformity of propertieswithin the material leaving this mixer.

[0039] The static mixer is preferably a device which is speciallydesigned for blending highly viscous fluids and commonly used inprocesses of injection-moulding of plastics, for example a static mixeras disclosed in patent U.S. Pat. No. 5,564,827. In general, this type ofmixer comprises static blending elements in a single piece, that is tosay without welds or joints, so as to avoid as far as is possible anydeformations and/or ruptures inside the mixer, even when the material tobe blended is highly viscous and thus requires high extrusion pressures.

[0040] It is important to emphasize that the use of a static mixer inthe process according to the present invention does not involve anydrawbacks relating to the handling of the plant or the quality of thecoating obtained. Specifically, the material which passes through thestatic mixer contains no crosslinking agents, and thus, unlike thecrosslinkable materials commonly used for coating power cables, does notgive rise to scorching phenomena due to the presence of possible zonesof stagnation of the material inside the static mixer.

[0041] The subsequent depositing and shaping step of the thermoplasticmaterial around the conductor can be carried out according to knowntechniques, by using an extrusion head of conventional type. Preferably,the extrusion head is a triple head, so as to achieve a co-deposition ofthe three coating layers of the conductor (inner semiconductive layer,insulating layer and outer semiconductive layer) which are typicallypresent in a medium-tension and/or high-tension cable.

[0042] In a second aspect, the present invention relates to a method forenhancing the electrical properties, in particular the dielectricrigidity, of a thermoplastic material comprising at least onethermoplastic polymer and at least one dielectric liquid, said methodcomprising the steps of: adding at least one dielectric liquid to thethermoplastic polymer, and passing said at least one thermoplasticpolymer, to which said at least one dielectric liquid has been added,through at least one static mixer.

[0043] According to one preferred embodiment of the present invention,the thermoplastic material comprises a polyolefin.

[0044] Said polyolefin preferably has an elastic flexural modulus,measured according to ASTM standard D790 at ambient temperature, of from30 to 1400 MPa and preferably from 60 to 1000 MPa.

[0045] Said polyolefin preferably has a melt flow index (MFI), measuredat 230° C. under a 21.6 N load according to ASTM standard D1238/L, offrom 0.05 to 10.0 dg/min and more preferably of from 0.5 to 5.0 dg/min.

[0046] Polyolefins that are-suitable for this purpose can preferably bechosen from:

[0047] (a) a high-density polyethylene (HDPE) with a density generallyof between 0.93 g/cm³ and 0.96 g/cm³;

[0048] (b) a propylene homopolymer or a copolymer of propylene with atleast one olefinic comonomer chosen from ethylene and an α-olefin otherthan propylene, said homopolymer or copolymer having a melting pointgreater than or equal to 140° C., preferably from 145 to 170° C., and aheat of fusion of from 30 to 100 J/g, preferably from 30 to 85 J/g.

[0049] When a copolymer of propylene with an olefinic comonomer is used,said comonomer is preferably present in an amount of less than or equalto 15 mol %, more preferably less than or equal to 10 mol %. Theolefinic comonomer is, in particular, ethylene or an α-olefin of formulaCH₂═CH—R, in which R is a linear or branched alkyl containing from 2 to10 carbon atoms said α-olefin being chosen, for example, from: 1-butene,1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-decene and1-dodecene, and the like, or combinations thereof. Propylene/ethylenecopolymers are particularly preferred.

[0050] According to one particularly preferred embodiment, thethermoplastic polymer is a polyolefin chosen from:

[0051] (1) a propylene homopolymer or a copolymer of propylene with atleast one olefinic comonomer chosen from ethylene and an α-olefin otherthan propylene, having an elastic flexural modulus generally of between30 and 900 MPa and preferably between 50 and 400 MPa;

[0052] (2) a heterogeneous copolymer comprising a thermoplastic phasebased on propylene and an elastomeric phase based on ethylenecopolymerized with an α-olefin, preferably with propylene, in which theelastomeric phase is present in an amount of at least 45% by weightrelative to the total weight of the heterogeneous copolymer.

[0053] The homopolymers or copolymers falling within category (1)exhibit a homogeneous microscopic structure, i.e. a structure which issubstantially free of heterogeneous phases dispersed in moleculardomains greater than one micron. Specifically, said materials do notexhibit the optical phenomena typical of heterogeneous polymermaterials, and in particular are characterized by improved transparencyand reduced “stress whitening” of the material due to localizedmechanical stresses.

[0054] Within category (1) mentioned above, particularly preferred is apropylene homopolymer or a copolymer of propylene with at least oneolefinic comonomer chosen from ethylene and an α-olefin other thanpropylene, said homopolymer or copolymer having:

[0055] a melting point of from 140 to 165° C.;

[0056] a heat of fusion of from 30 to 80 J/g;

[0057] a fraction which is soluble in boiling diethyl ether, in anamount of less than or equal to 12% by weight, preferably from 1 to 10%by weight, having a heat of fusion of less than or equal to 4 J/g andpreferably less than or equal to 2 J/g;

[0058] a fraction which is soluble in boiling n-heptane, in an amount offrom 15 to 60% by weight and preferably from 20 to 50% by weight, havinga heat of fusion of from 10 to 40 J/g and preferably from 15 to 30 J/g;and

[0059] a fraction which is insoluble in boiling n-heptane, in an amountof from 40 to 85% by weight and preferably from 50 to 80% by weight,having a heat of Fusion of greater than or equal to 45 J/g andpreferably from 50 to 95 J/g.

[0060] Further details regarding said materials and their use forcoating cables are given in European patent application No. 99122840filed on Nov. 17, 1999 in the name of the Applicant, which isincorporated herein by reference.

[0061] The heterogeneous copolymers falling within category (2) arethermoplastic elastomers obtained by block copolymerization of: (i)propylene, optionally containing smaller amounts of at least oneolefinic comonomer chosen from ethylene and an α-olefin other thanpropylene; and then of: (ii) a blend of ethylene with an α-olefin, inparticular propylene, and optionally with smaller portions of a diene.Said category of products is also commonly known as “reactorthermoplastic elastomers”.

[0062] Within category (2) mentioned above, particularly preferred is aheterogeneous copolymer in which the elastomeric phase consists of anelastomeric copolymer of ethylene and propylene which comprises from 15to 50% by weight of ethylene and from 50 to 85% by weight of propylene,relative to the weight of the elastomeric phase. Further detailsregarding said, materials and their use for coating cables are given inpatent application WO 00/41187 in the name of the Applicant, which isincorporated herein by reference.

[0063] Products of category (1) are commercially available, for example,under the brand name Rexflex® from Huntsman Polymer Corp.

[0064] Products of category (2) are commercially available, for example,under the brand name Hifax® from Montell.

[0065] The base thermoplastic polymer as described above can be used asa mechanical blend with a polymer of low crystallinity, generally with aheat of fusion of less than 30 J/g, the main function of which is toincrease the flexibility of the material. The amount of polymer of lowcrystallinity is generally less than 70% by weight, preferably between20% and 60% by weight, relative to the total weight of the thermoplasticmaterial.

[0066] The polymer of low crystallinity is preferably a copolymer ofethylene with an α-olefin containing from 3 to 12 carbon atoms, andoptionally with a diene. The α-olefin is preferably chosen from:propylene, 1-hexene and 1-octene. When a diene comonomer is present,this generally contains from 4 to 20 carbon atoms and is preferablychosen from: linear conjugated or non-conjugated diolefins, for example1,3-butadiene, 1,4-hexadiene or 1,6-octadiene, or mixtures thereof, andthe like; monocyclic or polycyclic dienes, for example1,4-cyclohexadiene, 5-ethylidene-2-norbornene, 5-methylene-2-norbornene,5-vinyl-2-norbornene, or mixtures thereof, and the like.

[0067] Among the ethylene copolymers that are particularly preferredare:

[0068] (i) copolymers having the following monomer composition: 35-90mol % of ethylene; 10-65 mol % of an α-olefin, preferably propylene;0-10 mol % of a diene, preferably 1,4-hexadiene or5-ethylidene-2-norbornene (EPR and EPDM rubbers fall within thiscategory);

[0069] (ii) copolymers having the following monomer composition: 75-97mol %, preferably 90-95 mol %, of ethylene; 3-25 mol %, preferably 5-10mol %, of an α-olefin; 0-5 mol %, preferably 0-2 mol %, of a diene (forexample ethylene/1-octene copolymers, such as the Engage® products fromDow-DuPont Elastomers).

[0070] The dielectric liquid which can be used to carry out the presentinvention is an aromatic and/or aliphatic oil generally having adielectric constant of less than or equal to 8, preferably less than 3.5(measured at 25° C. according to IEC standard 247).

[0071] The dielectric liquid preferably comprises:

[0072] (i) an alkylaryl hydrocarbon containing at least two, preferablyat least three, non-fused aromatic rings and having a ratio between thenumber of aryl carbon atoms and the total number of carbon atoms ofgreater than or equal to 0.6, preferably greater than or equal to 0.7,as disclosed in the co-pending European patent application No.00113661.3, filed on Jan. 28, 2000 in the name of the Applicant, whichis incorporated by reference; or

[0073] (ii) a diphenyl ether, which is unsubstituted or substituted withat least one linear or branched aliphatic, aromatic or mixed aliphaticand aromatic hydrocarbon radical, containing from 1 to 30 carbon atoms,preferably from 1 to 24 carbon atoms, as disclosed in the co-pendingEuropean patent application No. 00121110.1, filed on Sep. 28, 2000 inthe name of the Applicant, which is incorporated herein by reference,

[0074] or mixtures of (i) and (ii).

[0075] Even more preferably, the dielectric liquid according to theinvention comprises at least one alkylaryl hydrocarbon containing atleast three non-fused aromatic rings in an amount of not less than 10%by weight, relative to the total weight of the dielectric liquid.

[0076] Examples of alkylaryl hydrocarbons belonging to category (i)which can be used according to the present invention are: benzyltoluene,benzylxylene, (methylbenzyl)toluene, (methylbenzyl)xylene,dibenzyltoluene, dibenzylxylene, di(methylbenzyl)toluene,di(methylbenzyl)xylene, and the like, or mixtures thereof.

[0077] Examples of diphenyl ethers belonging to category (ii) which canbe used according to the present invention are: phenyl tolyl ether,2,3′-ditolyl ether, 2,2′-ditolyl ether, 2,4′-ditolyl ether, 3,3′-ditolylether, 3,4′-ditolyl ether, 4,4′-ditolyl ether, octadecyl diphenyl ether,and the like, or mixtures thereof.

[0078] The dielectric liquid which can be used to carry out the presentinvention has a predetermined viscosity such as to avoid rapid diffusionof the liquid through the insulating layer and thus outward migration ofsaid liquid, and at the same time such as to allow it to be easily fedand blended into the polymer material. Generally, the dielectric liquidhas a kinematic viscosity of from 1 to 500 mm²/s and preferably from 5to 100 mm²/s (measured at 20° C. according to ISO standard 3104).

[0079] According to a further preferred aspect, the dielectric liquidhas a hydrogen-absorbing capacity of greater than or equal to 5 mm³/minand preferably greater than or equal to 50 mm³/min (measured accordingto IEC standard 628-A).

[0080] According to a preferred aspect, to the dielectric liquid whichis suitable for carrying out the present invention can be added,generally in an amount of less than or equal to 1% by weight relative tothe weight of the liquid, an epoxy resin, which serves mainly to reducethe migration speed of the ions in an electric field, and thus thedielectric losses of the insulating material.

[0081] The weight ratio between the dielectric liquid and thethermoplastic polymer according to the invention is generally between1:99 and 25:75, preferably between 2:98 and 20:80 and even morepreferably between 3:97 and 15:85.

[0082] To carry out the process according to the invention, otherconventional components can be added to the thermoplastic material, forexample antioxidants, processing coadjuvants and water-tree retardants,and the like.

[0083] Conventional antioxidants that are suitable for this purpose are,for example, distearyl thiopropionate and pentaerythrityltetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and the like,or mixtures thereof.

[0084] Processing coadjuvants which can be added to the polymer baseinclude, for example, calcium stearate, zinc stearate, stearic acid,paraffin wax, and the like, or mixtures thereof.

[0085] When it is desired to prepare a semiconductive layer, aconductive filler, in particular carbon black, is generally dispersed inthe thermoplastic material, in an amount such as to give said materialsemiconductive properties (i.e. such as to obtain a resistivity of lessthan 5 Ohm*m at ambient temperature). This amount is generally between5% and 80% by weight, preferably between 10% and 50% by weight, relativeto the total weight of the mixture.

[0086] The possibility of using the same type of thermoplastic materialboth for the insulating layer and for the semiconductive layers isparticularly advantageous in the production of medium-tension orhigh-tension cables, since it ensures optimal adhesion between adjacentlayers and thus better electrical behaviour, especially at the interfacebetween the insulating layer and the inner. semiconductive layer, inwhich the electric field and thus the risk of partial discharges arehigher.

[0087] For the purposes of the present invention, the term“medium-tension” generally means a voltage of between 1 and 35 kV, while“high-tension” means voltages of greater than 35 kV.

[0088] Although the present description is mainly focused on theproduction of medium-tension or high-tension power transmission ordistribution cables, the process according to the present invention canbe used to prepare the insulating coating of electrical devices ingeneral. In particular, the process can be used to produce cables ofdifferent type, for example low-tension cables, telecommunicationscables or combined power/telecommunications cables, or to producecomponents of accessories used in the production of power lines, such aselastic sleeves for terminals or joints.

[0089] Further details will be illustrated by the following detaileddescription, with reference to the attached drawing, in which:

[0090]FIG. 1 is a perspective view of an electrical cable which isparticularly suitable for medium tension or high tension, and which canbe prepared according to the present invention;

[0091]FIG. 2 is a partial top-plan view of a production line accordingto the invention;

[0092]FIG. 3 is a partial cross section of an insulating layer of aDower cable, in which cross section are shown the points where theconcentration of the dielectric liquid was determined.

[0093] In FIG. 1, the cable 1 comprises a conductor 2; an inner layer 3with semiconductive properties; an intermediate layer 4 with insulatingproperties; an outer layer 5 with semiconductive properties; a metalshield 6; and an outer sheath 7.

[0094] The conductor 2 generally consists of metal wires, preferablycopper or aluminium wires, braided together according to conventionaltechniques.

[0095] At least one coating layer chosen from the insulating layer 4 andthe semiconductive layers 3 and 5 comprises the composition of theinvention as defined above.

[0096] Around the outer semiconductive layer 5 is usually placed ashield 6, generally consisting of electrically conductive, helicallywound wires or tapes. Said shield is then covered with a sheath 7,consisting of a thermoplastic material, for example non-crosslinkedpolyethylene (PE) or, preferably, a propylene homopolymer or copolymeras defined above.

[0097] The cable can moreover have an outer protective structure (notshown in FIG. 1) whose main function is to protect the cable againstmechanical impacts and/or compressions. This protective structure canbe, for example, metal armouring or a layer of expanded polymermaterial, as disclosed in patent application WO 98/52197. In general,this outer protective structure is in a radially internal positionrelative to the outer sheath 7.

[0098]FIG. 2 diagrammatically represents a plant 100 in accordance withone particular embodiment of the process according to the presentinvention.

[0099] In detail, the plant 100 illustrated in FIG. 2 mainly comprises:an extruder 10, a line 20 for supplying the dielectric liquid, a device90 for feeding the dielectric liquid to the extruder 10, a static mixer40 and an extrusion head 50, from the outlet of which (arrow A), inaccordance with the embodiment illustrated, the so-called “core” of thecable, that is to say the combination of the conductor 2, innersemiconductive layer 3, insulating coating 4 and outer semiconductivelayer 5 of the cable 1 in FIG. 1, is obtained.

[0100] The extruder 10, shown diagrammatically, comprises a barrel 11inside of which, via a suitable motor means 12, is rotated a screw 13provided to process and plasticize the thermoplastic polymer materialwith which a predetermined coating of the electrical cable 1 is made.

[0101] Said polymer material is introduced into the extruder 10 via afeed pipe 14, for example a hopper, and subjected to processing bypassing said material into the space between the inner surface of thebarrel 11 and the outer surface of the screw 13.

[0102] The extruder 10 moreover has a plurality of heating units 15distributed along the length of the screw 13, which provide the amountof heat required to plasticize the abovementioned polymer material, i.e.to bring it to the molten state.

[0103] In the specific embodiment illustrated in FIG. 2, the extruder 10comprises a further processing unit 17 into which the dielectric liquidis fed. This further processing unit 17 is connected to the extruder 10by one or more flanges 16.

[0104] As mentioned above, the plant 100 also includes a device 90 forfeeding in the dielectric liquid. Said device 90 preferably comprises atleast one injector. Even more preferably, said device 90 comprises atleast one pair of injectors, so as to distribute the dielectric liquidas homogeneously as possible in the molten polymer material.

[0105]FIG. 2 diagrammatically illustrates a device 90 comprising threeseparate injectors located on the same cross section of the extruder 10.Preferably, said three injectors are positioned on the same crosssection of the extruder 10 so as to be 120° away from each other.

[0106] The line 20 for supplying the dielectric liquid will be describedin detail hereinbelow in the present description.

[0107] Downstream of the device 90 for feeding in the dielectric liquid,the plant 100 advantageously has a filtration section 60 which, asmentioned above, has the purpose of removing any impurities contained inthe thermoplastic polymer material, the presence of which could cause adecrease in the electrical properties of the coating being produced.

[0108] In accordance with the present invention, the plant 100 moreoverincludes a static mixer 40 whose function is to optimize the mixing ofthe dielectric liquid into the thermoplastic material such that saiddielectric liquid can be uniformly distributed throughout the thicknessof the coating to be produced.

[0109] Finally, downstream of the static mixer 40, the plant 100includes an extrusion head 50 provided to shape one or more coatings ofthermoplastic polymer material around the conductor, the number of saidcoatings depending on the type of cable being processed.

[0110] For example, when the plant 100 in FIG. 2 is intended for theproduction of the cable 1 shown in FIG. 1, the conductor 2 must be fedthrough said extrusion head 50, and is generally unwound from a feedreel (not shown in FIG. 2) placed on the line, on which conductor thecombination of the inner semiconductive layer 3, the insulating layer 4and the outer semiconductive layer 5 is deposited, said combinationbeing technically defined by the term “core” of the cable 1.

[0111] In order to deposit the abovementioned “core”, the extrusion head50 is advantageously a triple extrusion head, which means that therecome together inside it, not only the conductor 2 but also threeseparate extrusion lines for processing the material which, oncedeposited on said conductor by means of the preshaping imparted by saidhead, will lead to the formation of the inner semiconductive layer, theinsulating coating and the outer semiconductive layer constituting theabovementioned “core” of the cable.

[0112] In the embodiment illustrated in FIG. 2 and provided for theproduction of the cable 1 in FIG. 1, the extruder 10 of the plant 100 isprovided for the processing, according to the present invention, of thethermoplastic material constituting the insulating coating 4 of saidcable 1, while the arrows B and C generally indicate the confluence inthe triple extrusion head 50 of two separate extrusion lines thatproduce the inner 3 and outer 4 semiconductive layers respectively.

[0113] The arrow A in FIG. 2 indicates the exit from the plant 100according to the invention of the “core” of the cable 1 as definedabove.

[0114] In accordance with a different embodiment according to which, asmentioned above, the coating layer obtained by the process according tothe invention can also be one or both of the semiconductive layers, thelines for processing and plasticizing the material constituting theabovementioned inner semiconductive layer 3 and outer semiconductivelayer 5 (indicated diagrammatically by the arrows B and C) can beentirely analogous to the line shown in detail in FIG. 2 and describedabove with particular reference to the production of the insulatingcoating 4 of the cable 1.

[0115] Generally, the “core” of the cable thus obtained, leaving theextrusion head 50, is subjected to a cooling step which can be carriedout, for example, by passing the abovementioned core through a coolingchannel, in which is placed a suitable fluid, typically well water orwater cooled to a temperature of about 12-15° C.

[0116] After a drying step, the “core” of the cable is usually subjectedto successive steps of coating with other elements typically present ina power cable.

[0117] In particular, with reference to the cable in FIG. 1, the “core”of the cable is stored on a suitable reel and conveyed to a line toapply the metal shield 6. This shield is generally obtained by means ofa tape screening machine, which helically places thin strips of copper(about 0.1-0.2 mm thick), via suitable rotating heads, preferably byoverlapping-the edges of said strips of about 33% of their surface.Alternatively, the metal shield consists of a plurality of copper wireshelically applied onto the cable core.

[0118] The cable 1 is then completed by applying, for example byextrusion, the outer polymer sheath 7 placed over the metal shield 6.

[0119]FIG. 2 moreover shows a possible layout of the line 20 forsupplying the dielectric liquid which forms a part of the plant 100according to the present invention.

[0120] In greater detail, said line 20 comprises a first feed tank 21 inwhich is stored, and refilled as it is consumed, the dielectric liquidused in the plant 100. Said tank 21 is connected, via a line 22, to asecond working tank 23.

[0121] The presence of two different tanks is particularly advantageoussince it makes possible to feed the line 20 with the dielectric liquidat a substantially constant working pressure. Specifically, wheneverfresh dielectric liquid is fed into the first tank 21 to top up thelevel, the pressure inside said first tank needs to be brought to thedesired working value. The presence of a second tank 23, not connectedto the first tank 21, thus makes it possible to have in said second tank23 dielectric liquid always at the working pressure, said dielectricliquid at the required pressure being transferred from the first tank 21to the second tank 23 only when the first tank 21, once loaded, has beenbrought to the desired working pressure.

[0122] In order to ensure a suitable regulation of the dielectric liquidflow to be released into the line 20, the second tank 23 is providedwith a suitable instrumentation, such as a manometer 24 and athermocouple 25, as well as a level-measuring device (not shown) and anexhaust valve (not shown) which is automatically actuated in the eventof there being an excess pressure inside the second tank 23.

[0123] The dielectric liquid leaving the second tank 23 is fed into apump 26 actuated by a motor means 27. Said pump 26 is preferably amembrane pump.

[0124] Advantageously, and as illustrated in FIG. 2, the pump 26 hasthree separate pumping heads 26′. Each pumping head 26′, provided with aseparate inlet line 28 and with a separate outlet line 29, is intendedto make the dielectric liquid flow towards the feed device 90 mentionedabove. As represented diagrammatically, said device 90 consists of threeseparate injectors, each of which is connected to a different outletline 29 of the pumping heads 26′.

[0125] Each outlet line 29 is also provided with a manometer 30 (tomonitor the pressure of the dielectric liquid in the line), a non-returnvalve 31 and a valve 32, the latter valve being intended to separate theline 20 from the rest of the plant 100.

[0126] In greater detail, the presence of said valve 32 on each line 29ensures that, especially when the plant 100 is started, the dielectricliquid is fed into the device 90 at the correct working pressure.Specifically, by closing the valves 32 on each line 29 and opening thevalve 33 located on the recycling line 34 toward the second tank 23, theline 20 is separated from the rest of the plant 100. This operatingsituation is maintained until the pressure of the dielectric liquidreaches the desired value, at which time the valve 33 can be closed andthe valves 32 can be opened.

[0127] When it is desired to produce a multipolar cable, the processhitherto described for a unipolar cable can be appropriately modified onthe basis of the indications given and the technical knowledge of aperson skilled in the art.

[0128] A number of preparation examples will now be given to describethe invention in further detail.

EXAMPLE 1

[0129] A medium-tension cable of the type illustrated in FIG. 1 wasproduced.

[0130] The production line used had the configuration illustrated inFIG. 2, and comprised three separate extruders flowing together in atriple extrusion head so as to obtain the co-deposition of thesemiconductive coatings and of the insulating coating to form the cablecore.

[0131] Into the downstream zone of the extruder used to deposit theinsulating layer were inserted three injectors positioned on the samecross section at 120° from each other, connected as illustrated in FIG.2 to a line for feeding in the dielectric liquid.

[0132] At the extruder outlet, downstream of the filtration section, wasplaced a static mixer for injection-moulding use, from the companySulzer, model SMK-R 30, having an inside diameter of 30.1 mm, an outsidediameter of 45 mm and comprising 4 mixing elements in series with atotal length of 135.5 mm.

[0133] By using this plant, a Cu/Sn conductor (consisting of a pluralityof wires braided together to form a cross section of 70 mm²) was coatedwith:

[0134] an inner semiconductive layer 0.5 mm thick;

[0135] an insulating layer 5.5 mm thick;

[0136] an outer semiconductive layer 0.5 mm thick.

[0137] The material of which both the semiconductive layers was made hadthe following composition: Hifax ® KS 081 100 phr Carbon black Y-200  55phr Jarylec ® EXP3  10 phr Irganox ® 1330  0.4 phr

[0138] in which:

[0139] Hifax® KS 081: heterogeneous propylene copolymer, with a contentof ethylene/propylene elastomeric phase equal to about 65% by weight(72% by weight of propylene in the elastomeric phase), a heat of fusionof 32 J/g, a melting point of 163° C., a MFI of 0.8 dg/min and aflexural modulus of about 70 MPa (commercial product from Montell);

[0140] Jarylec® EXP3: dibenzyltoluene (DBT) (commercial product from ElfAtochem);

[0141] Black Y-200: acetylene carbon black from the company SN2A, with aspecific surface area of 70 m²/g;

[0142] Irganox® 1330:1,3,5-trimethyl-2,4,6-tris-(3,5-di-tert-butyl-4-hydroxybenzyl)benzene(antioxidant from Ciba Geigy).

[0143] The term “phr” means parts by weight of each component per 100parts by weight of polymer.

[0144] This material was prepared by mixing the components together in aWerner internal mixer (internal volume: 6000 cm³) for 10 min at 200° C.(rotor speed: 44 rpm).

[0145] A 45 mm Bandera single-screw extruder in configuration 20 D,having three zones of temperature regulation with diathermic oil, wasused for the inner semiconductive layer, and a 60 mm Banderasingle-screw extruder in configuration 20 D was used for the outersemiconductive layer. The temperature profile of each extruder is givenin Table 1.

[0146] The insulating layer consisted of a thermoplastic materialcomprising Rexflex® WL105 and 7.5% by weight of Jarylec® EXP3,

[0147] in which:

[0148] Rexflex® WL105: propylene-homopolymer, having a melting point of160° C., a heat of fusion of 56.7 J/g, a MFI of 1.8 dg/min and anelastic flexural modulus of 290 MPa (commercial product from HuntsmanPolymer Corp.);

[0149] Jarylec® EXP3: as above.

[0150] The insulating layer was extruded by using a 100 mm Banderasingle-screw extruder in configuration 25 D, having a temperatureprofile as given in Table 1. TABLE 1 Inner Outer semiconductiveInsulating semiconductive Zone of the layer layer layer extruder (° C.)(° C.) (° C.) Zone 1 170 150 160 Zone 2 180 170 170 Zone 3 190 180 180Zone 4 — 180 190 Zone 5 — 190 — Extruder/head 200 190 200 flange Die 190

[0151] The following tests were carried out on the cable thus obtained.

[0152] Distribution of the Dielectric Liquid in the Insulating Layer.

[0153] Samples in the shape of slices (indicated by 70 in FIG. 3) 150 μmthick were cut from a cross-section of the cable by using a microtomeand were analysed by quantitative infrared spectroscopy (macro FTIR) inorder to determine the amount and the distribution of the dielectricliquid in the material. In particular, by using the typical absorptionbands of the dielectric liquid (aromatic rings at 696 cm⁻¹) and ofpolypropylene (alkyl branches at 901 cm⁻¹) as reference, calibrationcurves were used to determine the concentration of the dielectric liquidat, respectively:

[0154] four points (identified by the letters a-d in FIG. 3) placed at90° from each other and located on the same circumference 60 of theinsulating layer 4, and thus at the same distance from the conductor;

[0155] five points (identified by the letters e-h in FIG. 3) locatedadjacent to each other, about 1 mm apart, on the same radius definingthe thickness of the insulating layer 4.

[0156] The results obtained are given in Table 2.

[0157] Measurement of the Dielectric Rigidity of the Cable

[0158] From the cable obtained as above were cut three pieces, each witha useful length of 20 m. Said pieces were subjected to a test ofdielectric rigidity by using a voltage alternating at industrialfrequency (50 Hz), at ambient temperature. A gradually increasingvoltage was applied between the conductor and the earthed metal shield.In detail, by starting from an initial value of 0 kV, the voltage wasgradually increased every 10 min by an amount of 10 kV until perforationof the insulating layer occurred. The results of this test (as theaverage of the three pieces of cable) are given in Table 2.

EXAMPLE 2

[0159] A medium-tension cable was produced as described in Example 1,the only difference being that the insulating layer consisted of athermoplastic material comprising Hifax® KS 081 and 6.5% by weight ofJarylec® EXP3.

[0160] The same tests as in Example 1 were carried out on the cable thusproduced. The results are given in Table 2.

EXAMPLE 3

[0161] A medium-tension cable was produced as described in Example 1,the only difference being that the insulating layer consisted of athermoplastic material comprising Hifax® KS 081 and 9% by weight ofJarylec® EXP3.

[0162] A dielectric rigidity measurement was carried out on the cablethus produced, as described in Example 1. The results are given in Table2.

EXAMPLE 4 (COMPARATIVE)

[0163] A medium-tension cable was produced as described in Example 1,the only differences being that the production plant did not comprise astatic mixer and the amount of additive in the insulating material wasequal to 4% by weight.

[0164] The same tests as in Example 1 were carried out on the cable thusproduced. The results are given in Table 2.

EXAMPLE 5 (COMPARATIVE)

[0165] A medium-tension cable was produced as described in Example 1,the only differences being that the production plant did not comprise astatic mixer and both the insulating layer and the semiconductive layersdid not comprise dielectric liquid.

[0166] A measurement of dielectric rigidity was carried out on the cablethus produced, as described in Example 1. The results are given in Table2.

[0167] From the data given in Table 2, the following can be noted.

[0168] Firstly, it can be deduced that the process according to thepresent invention makes it possible to achieve a uniform distribution ofthe dielectric liquid both circumferentially relative to the cableconductor and radially in the thickness of the thermoplastic polymercoating comprising said dielectric liquid. Said result is not obtainedwhen (see Example 4) the process for producing this cable is carried outwithout a static mixer.

[0169] Secondly,, Table 2 demonstrates the relationship which existsbetween the dielectric rigidity and the distribution of the dielectricliquid: specifically, obtaining a uniform distribution of the dielectricliquid in the coating layer of the cable (see Examples 1 and 2)increases the dielectric rigidity of the cable. TABLE 2 EXAMPLE 1 2 3 4(*) 5 (*) Dielectric 45.3 53.6 61 28.9 28.2 rigidity (kV/mm)Distribution of the dielectric liquid (%) Point a 7.5 6.4 n.m. 1.2 — b7.2 6.7 4.0 — c 6.9 6.7 3.4 — d 7.2 6.7 3.9 — e 7.6 7.9 1.8 — f 7.8 6.73.2 — g 7.5 6.3 3.7 — h 7.3 6.5 4.4 — i 7.5 8.0 2.8 —

1. Process for producing a cable provided with at least onethermoplastic coating, which comprises: extruding a thermoplasticmaterial comprising at least one thermoplastic polymer and at least onedielectric liquid; passing said thermoplastic material through at leastone static mixer; a depositing and shaping said thermoplastic materialaround a conductor so as to obtain a layer of thermoplastic coating onsaid conductor.
 2. Process according to claim 1, in which saiddielectric liquid is added to said at least one thermoplastic polymer inthe molten state.
 3. Process according to claim 1, in which saiddielectric liquid is added to said at least one thermoplastic polymer inthe solid state.
 4. Process according to any one of the precedingclaims, in which said extrusion step comprises the following sub-steps:feeding said at least one thermoplastic polymer into at least oneextruder; a conveying said at least one thermoplastic polymer throughsaid at least one extruder; plasticizing said at least one thermoplasticpolymer travelling through said at least one extruder.
 5. Processaccording to claim 4, in which said dielectric liquid is added in a zoneof said at least one extruder in which said at least one thermoplasticpolymer is in the molten state.
 6. Process according to claim 5,characterized in that said dielectric liquid is added in at least twoseparate points of said zone of said at least one extruder.
 7. Processaccording to claim 4, in which said dielectric liquid is added to saidat least one thermoplastic polymer during said feeding sub-step. 8.Process according to claim 4, in which said dielectric liquid is addedto said at least one thermoplastic polymer before said feeding sub-step.9. Process according to claim 4, in which said dielectric liquid isadded in at least one zone of said at least one extruder in which saidat least one thermoplastic polymer is In the solid state.
 10. Processaccording to any one of the preceding claims, characterized in that italso comprises a filtration step of said thermoplastic material. 11.Process according to claim 10, in which said filtration step is carriedout prior to said step of passing said thermoplastic material through atleast one static mixer.
 12. Process according to claim 10, in which saidfiltration step is carried out after said step of passing saidthermoplastic material through at least one static mixer.
 13. Processaccording to any one of the preceding claims, characterized in that saidat least one thermoplastic coating is a layer of electrical insulation.14. Process according to any one of claims 1 to 12, characterized inthat said at least one thermoplastic coating is a semiconductive layer.15. Process according to any one of the preceding claims, characterizedin that said thermoplastic material comprises at least one polyolefin.16. Process according to claim 15, characterized in that said polyolefinhas an elastic flexural modulus, measured according to ASTM standardD790 at ambient temperature, of between 30 and 1400 MPa.
 17. Processaccording to claim 16, characterized in that said elastic flexuralmodulus is between 60 and 1000 MPa.
 18. Process according to claim 15,characterized in that said polyolefin has a melt flow index (MFI;),measured at 230° C. under a 21.6 N load according to ASTM standardD1238/L, of between 0.05 and 10.0 dg/min.
 19. Process according to claim18, characterized in that said melt flow index is between 0.5 and 5.0dg/min.
 20. Process according to any one of claims 15 to 19,characterized in that said polyolefin is chosen from the groupcomprising: a) a high-density polyethylene (HDPE) with a density ofbetween 0.93 g/cm³ and 0.96 g/cm³; b) a propylene homopolymer or acopolymer of propylene with at least one olefinic comonomer chosen fromethylene and an α-olefin other than propylene, said homopolymer orcopolymer having a melting point greater than or equal to 140° C. and aheat of fusion of from 30 to 100 J/g.
 21. Process according to any oneof the preceding claims, characterized in that said dielectric liquid isan aromatic oil, an aliphatic oil or an aliphatic and aromatic oil witha dielectric constant (measured at 25° C. according to IEC standard 247)of not greater than
 8. 22. Process according to claim 21, characterizedin that said dielectric constant is less than 3.5.
 23. Process accordingto claim 21, characterized in that said dielectric liquid comprises: (i)an alkylaryl hydrocarbon containing at least two, non-fused aromaticrings and having a ratio between the number of aryl carbon atoms and thetotal number of carbon atoms of greater than or equal to 0.6; or (ii) adiphenyl ether, which is unsubstituted or substituted with at least onelinear or branched aliphatic, aromatic or mixed aliphatic and aromatichydrocarbon radical, containing from 1 to 30 carbon atoms, or a mixtureof (i) and (ii).
 24. Process according to any one of the precedingclaims, characterized in that an epoxy resin is added to said dielectricliquid.
 25. Process according to claim 24, characterized in that saidepoxy resin is added in an amount of not greater than 1% by weightrelative to the weight of said dielectric liquid.
 26. Process accordingto any one of the preceding claims, characterized in that the weightratio between said dielectric liquid and said at least one thermoplasticpolymer is between 1:99 and 25:75.
 27. Process according to claim 26,characterized in that said weight ratio is between 2:98 and 20:80. 28.Process according to claim 27, characterized in that said weight ratiois between 3:97 and 15:85.
 29. Method for enhancing the electricalproperties, in particular the dielectric rigidity, of a thermoplasticmaterial comprising at least one thermoplastic polymer, said methodcomprising the steps of: adding at least one dielectric liquid to saidat least one thermoplastic polymer, passing said at least onethermoplastic polymer, to which said at least one dielectric liquid hasbeen added, through at least one static mixer.